A. Field of the Invention
The present invention relates generally to printing device, or more particularly relates to thermal printing device which prints on ordinary paper using an ink which melts when heated but is solid at room temperature.
B. Description of Prior Art
Various kind of printing devices are used for data processing in computer, facsimile and copying machine. Many efforts are directed to improve the device from the standpoint of its dimensions, printing speeds, noises and electric consumptions. Printing devices may be classified into two kinds, one of them is impact-type printer which effects printing by mechanical impacts, the other being non-impact-type without such mechanical impacts. Although the impact-type printer may have some advantages, the non-impact-type one is developing in recent years, because of its low noise and freedom of printing elements.
Non-impact-type printer may further be classified into inkjet-type printer, electrostatic-type printer and thermal printer. The inkjet-type spouts liquid ink in the form of minute ink drops from its nozzle and the ink drops are adhered onto printing paper as dots to effect printing. Electrostatic printer comprises a drum which can be charged electrostatically. Toner, i.e. a particle ink, is adhered onto the drum and is transcribed and fixed onto the printing paper to effect printing. Thermal printer, as is already mentioned, effects printing by using an ink-layer of heat-melting ink which can easily be transcribed onto the printing paper.
Characteristics advantage of these non-impact-type printers is that an ordinary paper can be used for printing. The ordinary paper without specific processing can easily be furnished without troublesome management of it.
By the way, inkjet-type printer has disadvantage that the jet nozzle is frequently choked, accordingly, it is difficult to effect printing steadily for a long time. It is necessary for the electrostatic printer to fix the electrostatic image after transcribing the charged particles, accordingly, fixing device is indispensable. For this reason, the printer tends to be large in its dimension and be expensive. Contrary to these printers, thermal printer has no such disadvantages.
In FIG. 1, a prior art terminal printing device is shown in which numeral 10 indicates an ink-roller on the surface of which minute uneveness is formed by attaching a foam-rubber or minute fibers on a rubber roller. Numeral 12 indicates a reforming roller of metal rod and the ink roller 10 and reforming roller 12 rotate in the direction of an arrow 14, being in contact with each other. The reforming roller 12 is heated by a heater 16 which is in spaced relationship with the roller 12 and the ink is melted which is in an ink pool 18 provided in contact with the ink roller 10. The ink is a mixture of coloured particle, printing medium such as paraffin compound and dispersion elements. The ink is solid below room temperature, but it melts rapidly when heated over predetermined degree of temperature and flows freely. The melted medium, i.e. the paraffin compound, can soak in the ordinary paper. The melted ink permeate into and adheres to the uneveness on the surface of ink-roller 10 and solidifies into ink-layer 20 in compliance with the fall of temperature. Numeral 22 indicates a thermal head with numerous minute heat elements inside. The thermal head is pressed toward the ink-roller 10 with a printing paper 26 with the aid of spring means 24 at the pressure of 200 to 1,000 gram/cm.sup.2. Numeral 28 indicates a paper feed roller and numeral 30 indicates a pulse motor for driving the paper feed roller 28.
In operation, the heat elements of thermal head 22 generate heat when a printing signal is fed to the thermal head 22 for melting the ink-layer 18 of ink roller 10 from the back of printing paper 26. The melted ink is then transcribed onto the surface of printing paper 26 as series of dots. After printing dots on the paper 26, the paper feed roller 28 is driven by pulse motor 30 to feed the paper 26 in the direction of an arrow 32 by a pitch for successively printing dots on the paper 26. The ink roller 10 is rotated in the direction of an arrow 14 by the frictional force from the paper 26 when the paper 26 is fed in the direction of an arrow 32. The surface of ink roller 10 needs to be remedied by supplying the ink from the ink pool 18, because uneveness of ink layer 20 occurs after printing dots on the paper 26.
In the above-described thermal printing device, dominant factors which decides printing quality are eveness of ink-layer and timely solidification and melting of ink. In other words, it is difficult to flatten the ink-layer, because the ink roller on which the ink-layer is formed has uneveness on its surface. Accordingly, the ink-layer does not uniformly come into contact with the thermal head, thereby the heat is not so well conducted to the ink-layer from the thermal head. Following that, the ink-layer can not sufficiently be melted, consequently the ink is not well transcribed onto the printing paper and the printing quality is degraded. Further, the printing quality becomes worse in case the solidification and melting of ink are not adequately conducted, resulting lack of dots and spotted paper.
Above all, the ink roller is the most important factor to obtain flattened ink-layer. Accordingly, the surface of ink-roller should be polished or numerous minute dents should be distributed uniformly all over the surface. But, such forming needs excellent technique and costs time and effort. Even if the surface is polished, the thickness of ink-layer becomes thinner, because the ink roller comes in close contact with the reforming roller. According to the experiments by the inventors of the present invention, the most suitable thickness of ink-layer amounts approximately from 20 to 35 microns. But, the thickness of ink-layer obtained by the combination of rubber ink roller and reforming roller amounts less than 10 microns, being insufficient for printing. Furthermore, it is almost impossible to increase the thickness of ink-layer by providing a gap between the ink roller and reforming roller due to the difficulty of levelizing the thickness. On the top of it, there are other problems to be solved in order to solidify and melt the ink timely, such as the distance between from the ink pool and the printing position and means for melting the ink.